The present invention relates to a stepping motor and a method for producing the stepping motor.
There is well known a stepping motor of a type having a stator with a plurality of coils stacked one on another along a shaft of the motor. For example, such the stepping motor is disclosed in a Japanese Unexamined Patent Application Publication No.63-39443. Each of the coils comprises a bobbin in a ring shape having a central shaft around which wires are wound, the central shaft having therein a hollow portion. A pair of yoke members having a plurality of teeth-shaped magnetic poles are fixedly secured to each of the coils such that the yoke members sandwich the coil therebetween. Thus, the one of the yoke members, the coil and the other one of yoke members are disposed in the motor along the motor shaft in this order. The yoke members are fixedly secured to the coil such that the teeth-shaped magnetic poles of the yoke members are inserted into the hollow portion of the central shaft of the coil so that each two adjacent teeth poles of one yoke member sandwich corresponding each tooth pole of the other yoke member.
A rotor is mounted in a space formed by the hollow portions of the central shafts of the bobbins stacked one on another. The rotor has a shaft and a permanent magnet fixedly secured thereto. The permanent magnet has a cylindrical shape with its axis extending along the shaft. The permanent magnet has a circular cross section on a plane perpendicular to its axis, and the circular cross section has a radius slightly smaller than that of the hollow portions of the central shafts of the bobbins.
The stator is produced in such a manner that the plurality of coils and the yoke members are assembled into one body and are wholly coated with resin so that the coils and the yoke members are integrally united into a stator. Therefore, the precision of the shape of thus produced stator and the positional accuracy of the members constituting the stator depend on the precisions of the shapes of the bobbins and the yoke members. However, the accuracies for processings of the yoke members and the coils are different from each other, so that the dimensional presicions of them are different from each other. It is very difficult to assemble, in high accuracy, the coils and the yoke members having different dimensional precisions from each other. Thus, a number of motors with their dimensional precisions lower than a desired precision are erroneously manufactured in the manufacturing procedure. As a result, the motor is made expensive. In order to solve the problem, it may be possible to process the bobbins of the coils and the yoke members with high accuracy. However, in such a case, the motor is still made expensive.